1. Field of the Invention
The present invention relates to novel granules or particulates comprised of silicate and silica, to a process for the production of such novel granules and to the use thereof in washing operations, and, more especially, to the use thereof in detergent compositions for automatic dishwashers.
2. Description of the Prior Art
Silicates, and especially metasilicate, are materials commonly used in detergent compositions for dishwashers; in fact, they impart the alkalinity required for washing dirty cooking utensils. In return, this alkalinity required for detergency applications presents safety problems. Indeed, detergent powders for dishwashers are in common domestic use and are often within the reach of children who are unaware of the hazards thereof and sometimes have a tendency to ingest them. The use of detergent powders which have a much lower alkalinity has also been sought for the past few years in order to ensure a greater safety to the user and to the population as a whole.
In addition to this safety requirement, silicates, and especially metasilicate, must meet other essential requirements, which often cannot be achieved in a simple manner. Thus, the silicate which constitutes an essential fraction of the detergent powder for dishwashers must be impeccable in appearance because, for detergency/washing applications, appearance often has a significant affect on marketability. The silicate grains must have a high degree of whiteness, a spherical shape and a uniform particle size such as to avoid sedimentation problems in the powder drum.
Other than these features relating to appearance, detergent compositions for dishwashers must be in concentrated form and, therefore, must consist of raw materials which have a high specific weight because the distribution magazines incorporated into dishwashers are typically of small size.
In addition to being concentrated, the raw materials must have a good stability during storage. Detergent compositions are sometimes stored for relatively long periods, under conditions of humidity and temperature which are often highly variable. Therefore, it is essential that after being stored for such time periods, the detergent composition remains in the form of a fluid powder, preferably free from agglomerates caused by caking phenomena. Above and beyond this physical stability, the detergent powder must be chemically stable. Thus, as a result of variations in humidity, the silicate must not degrade the more unstable constituents of the composition, such as perfumes, chlorinated salts and nonionic surfactants, such as polyoxyethylene glycols.
The totality of the aforesaid constraints, which are not always concordant, result in the fact that the ideal silicate is a sophisticated product, the production of which presents more and more complicated research problems. And in light of modern increasingly severe safety standards, priority will be given to decreasing alkalinity while at the same time avoiding deterioration in the other properties: whiteness, particle size, spherical shape, density and compatibility.
To date silicates used in detergent compositions for dishwashers are almost exclusively metasilicates, i.e., silicates in which the molar ratio SiO.sub.2 /Na.sub.2 O is equal to 1. This silicate is the most efficient because it has the highest alkalinity. From a chemical point of view, this metasilicate may be in the anhydrous form, the apparent density of which is very high, or in the form of the pentahydrate, the apparent density of which is lower. Nevertheless, both are caustic. A metasilicate granule having an anhydrous center or core, and a hydrated enveloping sheath or periphery has also been proposed to this art, in European Patent No. EP 100,700, but, like the metasilicates mentioned above, such granule does not have the properties of safety currently demanded by the market.
The preparation of a silicate of much lower alkalinity, and which is much less caustic in nature because it has a chemical composition in which the molar ratio SiO.sub.2 /Na.sub.2 O is in the vicinity of 2, is described in U.S. Pat. No. 3,687,640. This compound is aspherical in shape and has a density of approximately 0.8, which renders it insufficiently concentrated for use as a detergent. Moreover, it still has a high surface alkalinity, although it is lower than that of metasilicate. Furthermore, the process for the preparation of such silicate is difficult to carry out.